The present invention relates to a method and apparatus for testing the presence or absence of a defect in a spark plug insulator.
Hereinafter, the term “front” refers to a discharge side with respect to the axial (longitudinal) direction of an insulator when assembled into a spark plug and the term “rear” refers to a side opposite to the front side.
A spark plug includes an insulator, a center electrode disposed in an axial through hole of the insulator, a metal shell disposed around an outer periphery of the insulator and a ground electrode attached to a front end of the metal shell to define a discharge gap between the center electrode and the ground electrode. By the application of a high voltage to the center electrode, a spark discharge occurs within the discharge gap between the center electrode and the ground electrode. In the presence of a defect e.g. a pin hole in the insulator, however, there is a possibility the spark discharge does not occur properly due to discharge leak through the defect in the insulator. It is thus necessary to test the presence or absence of such a defect in the insulator before the actual use of the insulator.
Japanese Patent Publication No. 2550790 proposes a method for testing the presence or absence of a defect in a ceramic insulator by placing a first test electrode in an axial through hole of the ceramic insulator and a second test electrode on an outer peripheral side of the ceramic insulator and applying a potential difference between the first and second test electrodes. It is judged that there is no defect in the ceramic insulator in the occurrence of a so-called flashover phenomenon in which a spark discharge occurs between the test electrodes and passes through the opening of the axial through hole of the ceramic insulator. On the other hand, it is judged that there is a defect in the ceramic insulator when a spark discharge occurs between the test electrodes but does not pass through the opening of the axial through hole of the ceramic insulator due to discharge leak through the defect in the ceramic insulator.
In the above proposed insulator test method, the electric discharge through the defect in the insulator is likely to occur by the application of a larger potential difference between the test electrodes. Namely, it is effective to increase the potential difference between the test electrodes for improvement in test accuracy. However, the flashover occurs when the potential difference between the test electrodes reaches or exceeds a flashover voltage. There is a limit on the test accuracy improvement that can be achieved only by increasing the potential difference between the test electrodes in the above conventional insulator test method.
Japanese Laid-Open Patent Publication No. 2004-108817 proposes to conduct a defect detection test on a ceramic insulator under high-pressure conditions in a hermetically sealed container. As the high-pressure conditions lead to an increase in flashover voltage, the potential difference between the test electrodes can be increased to a higher value for test accuracy improvement, without causing a flashover phenomenon, under the high-pressure conditions.